1. Field of the Invention
This invention relates to solar energy collectors, and in one aspect, to an improved solar energy collector which is plastic, lightweight, semi-concentrating, nontracking and of the fluid circulating type for use in various heating applications.
2. Description of the Prior Art
Fluid circulating solar heating systems utilize a collector for changing the sun's energy to heat, a heat exchanger for transferring the heat energy to a circulating medium via connecting pipes, manifolds, circulating pumps, and the associated controls for such systems. A desirable feature for a solar energy collector is an efficient absorber of the solar energy. One such absorber is a dark vaned absorber disposed in a fluid circulating tube such that as the sun's energy is changed to heat energy on the absorber the circulating fluid will absorb the heat. Because of the use of plastic, however, it is necessary, in the event of power failure or when the system is allowed to drain down, that the absorber will not become too hot and melt the tube. Further, to avoid the necessity of tracking, the reflectors around the absorber are not parabolic reflectors having a focus, and the design of the system should be such that ambient heat around the absorbers cannot be easily lost by convective currents.
A typical heating element for solar energy is described in U.S. Pat. No. 1,575,309, issued to W. A. Anderson, Mar. 2, 1926. This patent discloses a solar heating element using a parabolic reflector, in the center of which was mounted a light absorbing heating member 21 positioned to absorb the sun's rays striking the reflector. The heating member 21 was placed in a transparent tube through which water could be circulated, resulting in the heating of the water by the heating member. A system of this nature would necessarily have to be supported to track the movement of the sun to be efficient more than a limited period each day.
Another similar patent teaching the use of an absorber within a transparent tube positioned at the focus of a reflector is U.S. Pat. No. 4,074,678, issued Feb. 21, 1978, to M. Posnansky.
The present invention utilizes a generally semi-cylindrical reflector positioned about each of a plurality of clear plastic tubes for the heat transfer medium. Such semi-cylindrical reflectors do not focus light striking the reflector to a predetermined point and thus tracking is unnecessary.
Another important part of an efficient, all plastic collector is the protection of the components from exceedingly high temperatures. This is necessary particularly if there is a power failure or the system is drained for other reasons. Protection of this sort is accomplished by the use of a light blocking system or light valve which has the effect of stopping the sunlight from reaching the absorber during such periods to avoid high "stagnation" temperatures. Prior art related to such "light valves" for solar collectors include U.S. Pat. No. 4,261,331, issued Apr. 14, 1981, to R. B. Stevens. This patent discloses a heat-sensitive optical switch which is operative when the temperature of the heat transfer fluid gets above a predetermined value. A precipitate forms in the fluid, causing a light scattering layer to appear between the sun and the absorber. U.S. Pat. No. 4,148,563, issued Apr. 10, 1979, to E. Herbert discloses two mating collector elements utilizing V-shaped ridges with a 90 degree included angle. When a film of liquid is disposed between the mating surfaces with the V-shaped ridges, light is allowed to pass and heat up the liquid, but when the liquid reaches a certain temperature the liquid vaporizes, causing the incoming light to be reflected by the V-shaped ridges or prisms. Another piece of prior art which is specifically related to the "light valve" is U.S. Pat. No. 4,056,094, issued Nov. 1, 1977, to P. Rosenberg. This patent shows a valve for a flat plate collector utilizing rows of V-shaped ridges on the light transmitting panel which is spaced from the absorber plate and through which space the heat transfer fluid is caused to flow. When the fluid is in the space the light is transmitted and when the fluid is drained away, the V-shaped ridges cause total internal reflection of the light so it is not allowed to pass through the transmitting panel to the absorber. The use of linear V-shaped ridges or prisms with the facets at 45.degree. with the top surface require frequent adjustment of a flat plate solar collector throughout the year, see Column 8, lines 28 to 48.
The present invention overcomes the deficiencies of the prior art and provides a highly efficient solar energy collector which has a unique automatic shutdown or light valve system to eliminate high stagnation temperatures, and one which is designed such that no tracking or adjustment or alignment with the sun is necessary to obtain its near maximum efficiency and utilize its protective features. The collector is usable with fluid circulating systems and is particularly compatible with "drain-back" hot water systems.